1. Field of the Invention
The present invention relates to a polyester composition and a process for producing the same. More particularly, the present invention relates to a polyester composition having enhanced melt-spinning and drawing properties and useful for producing polyester composition fibers having excellent mechanical properties, for example, tensile strength and Young's modulus, and a process for producing the polyester composition.
2. Description of the Related Arts
It is known that various types of polyester resins, particularly polyethylene terephthalate resins, have excellent chemical and physical properties and, therefore, are utilizable for various purposes, particularly fibers and films
In an ordinary process for producing the polyester resin, for example, polyethylene terephthalate resin, a glycol ester of terephthalic acid or its oligomer is produced by esterifying terephthalic acid with ethylene glycol or ethylene oxide, or by interesterifying dialkyl terephthalate with ethylene glycol, and then the esterification or interesterification product is subjected to a polycondensation process while a delusterant consisting of titanium dioxide particles is added to the polycondensation mixture.
The resultant polyester resin is subjected to a melt-spinning process wherein the polyester resin is melted and extruded through a melt-spinning nozzle to form a number of undrawn polyester filaments, and the undrawn polyester filaments are drawn. Recently, a new process wherein the melt-spun polyester filaments are taken up at a high speed of 2,000 m/min or more, and the resultant highly orientated polyester filaments (POY) are subjected to a draw-false twisting process, has become popular.
Further, European Unexamined Patent Publication No.. 95712 for Asahi Kasei Kogyo K. K. disclose a process for producing polyester fibers having satisfactory physical properties in actual use only by a melt-spinning step, wherein a melt of a polyester resin is melt-spun and taken up at extremely high speed of 5,000 m/min, particularly, 8,000 m/min.
However, the melt-spinning and taking up procedures at the extremely high speed of 5,000 m/min or more results in an increase in the number of breakages of individual filaments and/or filament yarns and the resultant polyester filaments yarns contain a number of fluffs consisting of broken individual filaments and, therefore, exhibit a degraded processability in the next processes.
The above-mentioned disadvantages become more serious with each decrease in the thickness of the individual filaments, and with each increase in the number of individual filaments per yarn. Accordingly, the high speed melt-spinning procedure at a taking up speed of 6,000 m/min or more is, in practice, very difficult to carry out without producing fluffs.
Due to recent developments, the polyester staple fibers and filaments having various functions are now produced at an enhanced productivity and, therefore, are required to have improved qualities, especially, improved mechanical properties, for example, tensile strength and Young's modulus.
In particular, in a direct melt-spinning process for producing practically usable polyester filaments only by a melt-spinning procedure, it is necessary to carry out the taking-up procedure at a high speed of at least 5,000 m/min. The mechanical properties such as tensile strength and Young's modulus of the resultant polyester filaments, however, are not always satisfactory in comparison with those produced by the ordinary melt-spinning-drawing process.
During research into ways of eliminating the above-mentioned disadvantages, one of the inventors of the present invention discovered that one reason for the breakage of individual filaments during the high speed melt-spinning procedure is that the crystallization of orientated polyester molecules is accelerated in the high speed melt-spinning procedure. Based on this discovery, Japanese Unexamined Patent publication Nos. 57-34123 and 57-42922 for Teijin Limited, discloses a method for restricting the crystallization of the polyester resin in the high speed melt-spinning procedure. In this method, p-hydroxybenzoic acid (POBA) is added to a polyester producing mixture at any stage before the polyester production is completed. In practice, it was found that the resultant polyester resin had a satisfactory color and was useful for the high speed melt-spinning process at a take-up speed of 4,000 to 5,000 m/min, with a decreased number of breakages of the melt-spun individual filaments. However, when a high speed melt-spinning procedure at a take-up speed of more than 5,000 m/min was applied to the polyester resin, it was found that the number of breakages of the individual filaments became undesirably large.
During research into a solution of the above-mentioned disadvantages, one of the inventors of the present invention discovered that the breakages of the individual filaments in the high speed melt-spinning procedure are influenced by the particle size and the distribution of titanium dioxide particles dispersed in a relatively large amount in the polyester resin, in addition to the influence of the accelerated crystallization of the polyester molecules. Based on this discovery, Japanese Unexamined Patent Publication No. 60-112849 for Teijin Ltd., disclosed that the high speed melt-spinnability of the polyester resin containing titanium dioxide particles is improved by using specific titanium dioxide particles having a small average particle size and a very sharp distribution curve of the particle size.
Although the above-mentioned specific titanium dioxide particles are effective for decreasing the breakage number of the individual filaments in the high speed melt-spinning procedure, nevertheless, the reduction in the number of breakages is still unsatisfactory.
Accordingly, with respect to the high speed melt-spinning procedure for polyester resin, it is strongly desired to provide a new type of polyester resin composition containing titanium dioxide particles and capable of producing polyester fibers or filaments having satisfactory mechanical properties with a satisfactory decrease in the number of breakages of the individual filaments.